A solid oxide fuel cell (SOFC) is a device that generates electricity by a chemical reaction. FIG. 1 shows a conventional SOFC cell 10 including a cathode layer 102, an anode layer 106, and an electrolyte layer 104. Fuel cells are typically characterized by their electrolyte material, with SOFCs having a solid oxide or ceramic electrolyte.
During operation of the SOFC, an oxidant, usually air, is fed through a plurality of air channels 120 defined by the cathode 102, while fuel, such as hydrogen gas (H2), is fed through a plurality of fuel channels 121 defined by the anode 106. The oxidant and fuel channels can be oriented at right angles to one another. The anode and cathode layers are separated by an electrolyte layer 104. During operation, the oxidant is reduced to oxygen ions at the cathode. These oxygen ions can then diffuse through the solid oxide electrolyte to the anode where they can electrochemically oxidize the fuel. In this reaction, a water byproduct is given off as well as two electrons. These electrons are transported through the anode to an external circuit (not shown) and then back to the cathode, providing a source of electrical energy in the external circuit. The flow of electrons in the external circuit typically provides an electrical potential of approximately 1.1 volts.
To generate larger voltages, a larger number of individual cells (each cell consisting of an anode and a cathode separated by an electrolyte layer) are combined in series so that the electricity each cell generates can be combined. During the manufacturing process, it is often desirable to combine a plurality of cells into a larger unit, referred to herein as a “cell unit” for convenience. FIG. 2 illustrates an exemplary embodiment of a solid oxide fuel cell unit 20. The cell unit 20 of FIG. 2 includes six separate cells (231, 232, 233, 234, 235, and 236) as shown in FIG. 1. As in the cell shown in FIG. 1, each of the individual cells combined to form the cell unit 20 of FIG. 2 includes a cathode layer 202 (having air channels 220) and an anode layer 206 (with fuel channels 221) separated by an electrolyte layer 204. Individual cells are combined using interconnect layers 208. These individually fabricated units having a particular power output can then be combined together to create a fuel cell stack with virtually any desired total power output. The desired number of individual cell units are stacked on top of each other and bonded together to create the final solid oxide fuel cell stack.
The bonding material used to connect the individual cell units should be electrically connective, gas permeable, mechanically strong, and thermally stable throughout the temperature ranges to which the fuel cell stack will be subjected. Due to the demanding operating conditions for a solid oxide fuel cell stack, an ideal bonding material that satisfies these requirements has proven elusive.
Therefore, there is a need for an improved bonding layer used to join individually formed cell units together to create a solid oxide fuel cell stack.
The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing.